The present invention refers to a seal device for sealing between a wall and a shaft, which extends through a hole in the wall, comprising an annular rotary symmetrical first seal element, which surrounds the shaft coaxially with the latter and which is connected to the wall, and an annular rotary symmetrical second seal element, which, is connected to the shaft coaxially with the latter. The seal elements are provided with respective annular seal surfaces, which extend in radial planes through the shaft and abut sealingly against each other, wherein at least one of the seal elements is attached directly to a holding member, which is connected to one of the wall and the shaft.
Such a seal device is used, inter alia, in pumps for high or low viscous liquids of different types, for example liquids of food, pharmaceuticals or the like, which liquids are sensitive to contamination. The seal elements are provided of a hard material, for example carbon or usually a ceramic, such as silicon carbide or hard metal. Such ceramic seal elements have a long lifetime and seals efficiently between the pump shaft and the pump housing, which contributes to a high reliability of pumps provided with said seal device. The long lifetime of the ceramic seal elements is of course due to the high resistance to wear of the ceramic material, but also to the fact that the liquid being pumped forms a thin lubricating film between the abutting seat surfaces of the seal elements.
However, it has been shown that it may arise hygienic problems in connection with pumping of certain liquids. Consequently, in connection with fastening of the annular seal element to the holding member which consists of a steel material, usually stainless steel, it is in practice impossible to avoid completely the emergence of cavities in the form of small openings and holes between the seal element and the holding member. Liquid residues may therefore in unfavourably cases be caught in said cavities and cause impermissible bacterial growth, which may result in contamination of the liquid, which is pumped.
As a solution to this hygienic problem it has previously been proposed that an annular rubber element is placed between the seal element and the holding member. The holding member may also be provided with drive pins, which extend into grooves in the seal element of the holding member. Certainly, such an annular rubber element seals efficiently between the seal element and the holding member, so that no cavities for liquid residues arise, but unfortunately gives rise to another problem. Consequently, it happens that pumps provided with seal devices of said type are used intermittently for the same kind of liquid without any intermediate washing, sterilisation or cleaning of the pumps which means that liquid, for example liquid food, during a period of rest of such a pump may get dry between the two seal elements abutting each other so that they get stuck to each other. This may result in cracking of the seal element in connection with the starting of the pump because it is brittle on the one hand and on the other hand resiliently connected to the holding member by means of said annular rubber element. Said drive pins may also cause cracks in the seal element.
The object of the present invention is to provide a seal device of the discussed type which remedies the above-described problems and which has a high reliability.
This object is achieved by a seal device of the initially described type which according to one aspect of the invention has the holding member connected to the wall and which is characterized in that the seal element of the holding member is provided with a circumferential surface, which extends in the hole of the wall so that an annular gap is formed between the wall and said circumferential surface, and that an elastic sealing ring is provided in said gap for sealing between the wall and the seal element of the holding member
Said object is also achieved by a seal device of the initially described type which according to another aspect of the invention has the holding member connected to the shaft and which is characterized in that the seal element of the holding member is provided with a circumferential surface, which extends in an annular coaxial recess in the shaft, wherein the recess radially outwardly is delimited by an inner circumferential surface of the shaft so that an annular gap is formed between the shaft and the circumferential surface of said seal element, and that an elastic sealing ring is provided in said gap for sealing between the shaft and said seal element.
Hereby, the advantage is achieved that the seal element is rigidly attached directly to the holding member without the risk of liquid residues being caught in cavities between the seal element and the holding member because said sealing ring prevents liquid from reaching said cavities.
According to a preferred embodiment of the seal device according to the invention said circumferential surface of the seal element of the holding member continues axially to a circumferential surface of the holding member via a joint between said seal element and holding member. The elastic sealing ring extends to the circumferential surface of the holding member and covers said joint, whereby liquid is prevented from penetrating the joint. Moreover, the holding member is provided with an outer circumferential groove and the elastic sealing ring is provided with a radially inwardly extending portion, Which is provided in said circumferential groove, whereby the sealing ring is fixedly arranged in the intended position. Advantageously, the elastic sealing ring has a P-shaped cross-section, wherein the head of the P is provided in said circumferential groove.
Preferably the holding member is provided with a portion, which surrounds the seal element of the holding member and which axially extends a distance along the latter, wherein said seal element is attached to said portion of the holding member for example by gluing, hard soldering or shrinking of the holding member onto said seal element, Advantageously, the seal element of the holding member may be designed with a rectangular cross-section.
However, it is preferred that the seal element of the holding member has a substantially L-shaped cross-section, wherein one of the legs of the L extends axially and the other leg of the L extends radially. Said portion of the holding member is complementary to said L-shaped cross-section and extends inwardly on the upper side of said axial leg.
The object of the invention is also achieved by a seal device of the initially described type which according to a further aspect of the invention is characterized in that the seal element of the holding member is provided with a first portion, which is directly attached to the holding member and a second portion, which forms one of said annular sealing surfaces and which extends radially outwardly from the first portion, wherein an annular outwardly open groove is formed between the second portion of the seal element and the holding member, and that an elastic annular gasket is provided in said groove. The annular gasket guarantees that liquid may not penetrate possible cavities between the holding member and the first portion of the seal element.
Preferably, the first portion of the seal element of the holding member is annular and coaxial with the shaft, wherein the holding member surrounds said first portion, which means that the seal element may be attached to the holding member by a shrinkage process.
Advantageously, the holding member forms an axial shoulder against which the second portion of the seal element abuts, which automatically gives the desired width of the annular groove in connection with assembly of the seal element on the holding member so that the elastic gasket seals in the groove.
Suitably, the holding member and the second portion of the seal element are provided with respective annular surfaces, which extend in radial planes through the shaft, wherein said annular groove is delimited by said annular surfaces.